Assuming you already use OO, Moose makes a lot of common tasks much easier. Writing object accessors (i.e. getter/setter methods) all the time gets tedious. Moose makes them trivial - and even adds type checking. So if your BankAccount class has a "current_balance" attribute, rather than writing something like this:

That way, whenever one of those methods is called, you see a warning about it. But for this to work, you'd need to go through your code and replace DB->new with DebuggingDB->new everywhere. But we can go better than that...

The ability to apply roles not just to classes, but to objects at run-time, is very powerful. This is something you can do without Moose, certainly, but Moose makes it a lot easier.

Lastly, it gives you some very powerful introspection: much more so than Perl's default OO introspection (isa, can and DOES).

Yes, Moose does have a performance hit. However, much of that is compile-time rather than run-time. So if you have a long-running process, such as a daemon, this is usually quite acceptable. (Whereas in a CGI script, where start-up time is important, Moose would probably be impractical.)

Moose isn't right for every project, but if you're doing OO, if start-up time isn't so important, and you don't care about adding some pretty heavy dependencies to your project, then it's a natural choice.